1. Field of the Invention
The present invention relates to method and apparatus for sterilization and/or pasteurization of consumables including water, liquid food or food submerged into liquids including juices, parenteral products, milk, yogurt, meat parts in water or other liquid, raw oysters in water, and so forth.
2. Description of Related Art
Use of shockwaves for sterilization is known. U.S. Pat. No. 5,588,357 to H. Tomikawa et al. discloses utilization of shock waves created by rapid electric discharge through a conducting wire in liquid for food sterilization. U.S. Pat. No. 6,264,543 discloses use of an array of electromechanical transducers to create shockwaves in liquid for the purpose of tenderization and sterilization of meat. U.S. Pat. No. 5,273,766 discloses use of solid explosives detonated in water for producing strong shockwaves for meat tenderization.
Food sterilization and pasteurization processes kill a significant number of harmful bacteria making food safe for consumption and extending its shelf life. Using shockwaves for sterilization and pasteurization of liquid food or food immersed in liquid allows food sterilization without significant elevation of temperature, which can affect food taste, texture and appearance.
Bacteria can be classified in two major groups, gram-positive and gram-negative. Gram-positive bacteria cell walls are simple in structure, but have thick peptidoglycan layers (10-20 layers thick) which make the cell walls strong and robust. Gram-negative cells have complex cell wall structures but much thinner peptidoglycan layers (only 1-2 layers thick). Therefore, the gram-positive cells are stronger, less likely to be broken mechanically, and are less permeable than the gram-negative cells. At the same time gram-positive cells, being larger, are more susceptible to sheer stress caused by a shockwave. Tensile strengths of cell walls of a number of bacteria were evaluated using rapid decompression experiments (see Carpita N. C., “Tensile Strength of Cell Walls of Living Cells”, Plant Physiol., Vol. 79, pgs. 485-488, (1985)). In these experiments it was found that liquid cultures of the bacterium Salmonella typhimurium (gram-negative) were disrupted at a pressure drop of ˜10 MPa. Blastocladiella emersonii (aquatic fungus), which has a wall thickness about two orders of magnitude larger than Salmonella, was disrupted with an even lower pressure drop of 6.5 MPa. These pressures are much lower than pressures used in the High Pressure Processing (HPP) method for sterilization of food products indicating that rapid compression/decompression, that will be typical for the shockwave effect, is an effective technique for sterilization. In a separate study it was also shown that Escherichia coli can be killed using an electrohydraulic shock-wave generator typically used for extracorporeal lithotripsy (see A. M. Loske et al., “Repeated application of shock waves as possible method for food preservation”, Shock Waves, Vol. 9, pgs. 49-55 (1999)). In this study it was determined that sterilization occurred only after multiple exposures to shock waves with the peak pressure of ˜50 MPa.
A number of techniques exist for generation of intensive shock waves in liquids. These methods include electric arc discharge, wire explosion, and projectile impact. The biomechanical effects of shock waves have been studied for the killing of bacteria and general food sterilization. See K. Teshima et al., “Biomechanical Effects of Shockwaves on Escherichia Coli and Aphage DNA”, Shock Waves, Vol. 4, pgs. 293-297 (1995); J. Barthel et al., “Biomechanical and Biochemical Cellular Response Due to Shock Waves”, Proc. 26th Army Science Conference, Orlando, Fla., (2008); and A. Abe et al., “The Effect of Shock Pressures on the Inactivation of a Marine Vibrio sp.”, Shock Waves, Vol. 17, pgs. 143-151 (2007).
However, despite the clearly demonstrated effectiveness of using high-intensity shock waves for killing bacteria, there are no practical applications of this effect. The main problem is lack of capability of creating shockwaves of needed intensity in industrial settings that is compatible with food or parenteral products processing. Use of electromechanical transducers will require shockwaves focused in a small volume to attain pressure levels required for killing bacteria, thus making sterilization of large amounts of food impractical. Use of exploding wire as disclosed in U.S. Pat. No. 5,588,357 is inefficient and requires a large capacitor as an energy source for rapid vaporization of the wire that creates the shockwave. Also food would need to be insolated in plastic to avoid contact with metal particles generated by the exploded wire.
Use of explosives, as disclosed in U.S. Pat. No. 5,273,766 for meat tenderizing, can generate strong shockwaves in large volumes. However, during processing, food needs to be insolated from toxic explosive products. Also, use of explosives in an industrial environment is not desirable because of safety and environmental concerns.
Thus, there is a critical need for efficient generation of high intensity shockwaves for food sterilization and pasteurization. The present invention contemplates elimination of the drawbacks associated with prior art of generation of shockwaves for sterilization and provides a method and apparatus for sterilization and pasteurization that is critical for practical application of shockwaves to sterilization and pasteurization. The present invention will also allow use of scalable sterilization apparatuses in households or in industry.
It is therefore the object of the present invention to provide a method and apparatus for sterilization or pasteurization of water, liquid food or food immersed in liquid such as juices, parenteral fluids, milk, shellfish, meat parts, ready to eat meals composed of cooked or raw produce, avocado paste, and similar products.
It is another object of the present invention to provide a method and apparatus for sterilization or pasteurization of food products without significant temperature rise during processing that can affect its nutritional or sensory quality.
A further object of the present invention is to provide a method and apparatus for sterilization or pasteurization that can be scaled for use in households or in industry.
Another object of the invention is to provide a method and apparatus that can be used for batch or continuous sterilization or pasteurization processing of food products.
Another object of the invention is to provide a method and apparatus for rapid and energy efficient sterilization of water and consumer products that can be immersed in liquid such as medical devices.